Recently, power IC's with a high breakdown voltage, which integrate high-breakdown-voltage devices such as diodes, insulated gate bipolar transistors (hereinafter referred to as "IGBTs") and MOSFET's with their driving, control and protection circuits on one silicon substrate, have been rapidly developed based on progress achieved in separation techniques such as junction-separation and dielectric-separation. In particular, the progress made with respect to the dielectric-separation technique, which combines the laminated wafer (hereinafter referred to as the "SOI wafer") and trench technique, has facilitated integrating a plurality of high-breakdown-voltage bipolar devices with a high breakdown voltage and unipolar devices with a high breakdown voltage and expanded the applications of the power IC's. For example, a totem-pole circuit which employs insulated gate bipolar devices with a high breakdown voltage such as IGBT's or an integrated circuit which connects such totem-pole circuits in tandem are formed on one single chip.
FIG. 34 is a circuit diagram of a conventional output circuit of a power IC with a high breakdown voltage including a totem-pole circuit. As shown in FIG. 34, the conventional output circuit includes a totem-pole circuit 2a including two n-channel MOSFETs N1 and N2 with a high breakdown voltage and a conventional level-shift circuit 1c which drives the gate of the MOSFET N1 on the high potential side. The totem-pole circuit 2a is widely applied to inverter IC's for driving motors, driver IC's for driving display devices and such IC's. The level-shift circuit 1c needs an independent low voltage power supply VL in addition to an n-channel MOSFET N7 with a high breakdown voltage, resistors with high resistance R2, R3, R6 and a p-channel MOSFET P3 with a low breakdown voltage.
FIG. 35 is a circuit diagram of another conventional output circuit of a power IC with a high breakdown voltage including a push-pull circuit. The push-pull circuit is different from the foregoing totem-pole circuit 2a in that the push-pull circuit includes a p-channel device with a high breakdown voltage on the high potential side, i.e. on the upper arm side, instead of the n-channel device with a high breakdown voltage. Referring now to FIG. 35, the p-channel device is a p-channel MOSFET P4 with a high breakdown voltage. By employing a p-channel device and an n-channel device, the reference potential of the power supply for gate drive may be made coincide with the potential of the output terminal and the driver circuit, including the level-shift circuit 1a, for driving the device on the upper arm may be simplified.
The conventional output circuit of FIG. 34 needs an independent power supply for the level-shift circuit thereof. The provision of the independent power supply does not pose any serious problem to IC's including a few output circuits such as a three-phase inverter IC. However, the provision of the independent power supply poses serious problems to IC's including many output circuits, such as a display driver IC, which includes several tens of output circuits. Although not illustrated, a charge-pump circuit may be used in substitute for the level-shift circuit, but it has the same problems discussed above. Since the totem-pole circuit employs the same n-channel devices on the upper arm and the lower arm (the low potential side), the so-called arm-short-circuit in which the upper and lower arms are simultaneously in the on-state thereof tends to be caused. Therefore, countermeasures for preventing the arm-short-circuit are necessary. In addition, countermeasures for making flow a reverse current, which flows through the devices constituting the totem-pole circuit depending on the state of load, are necessary especially when an IGBT is used for the device of the totem-pole circuit.
The current carrying capability of the p-channel MOSFET which constitutes the push-pull circuit of FIG. 35 is inferior to that of the n-channel device. Therefore, it is necessary to increase the current carrying area of the p-channel MOSFET on the upper arm for making the current of same magnitude flow through the p-channel MOSFET on the upper arm as well as through the n-channel MOSFET on the lower arm. The adverse effects of the widened current carrying area cancel the effects of the simplified gate driver circuit. As a result, the chip size for the push-pull circuit exceeds the chip size for the totem-pole circuit, which further causes chip cost increase.
In view of the foregoing, it is an object of the present invention to provide an output circuit for a power IC with a high breakdown voltage which obviates the foregoing problems. It is another object of the present invention to provide an output circuit for a power IC with a high breakdown voltage which does not need any independent power supply. It is still another object of the present invention to provide an output circuit for a power IC with a high breakdown voltage which facilitates preventing the arm-short-circuit from causing. It is a further object of the present invention to provide an output circuit for a power IC with a high breakdown voltage which facilitates making a reverse current flow and reducing the chip size.